[boost] [filesystem] adding path equality and path equivalence

From: Beman Dawes (bdawes_at_[hidden])
Date: 2004-02-01 11:12:07

Equality
--------

The path relational operators will be defined purely in terms of string 
comparison between the textual representation of the two paths. There are 
two ways to do this: (1) string comparison using std::string relational 
operators or (2) string comparison using std::lexicographical_compare.

Given two paths, "foo.bar" and "foo/bar", (1) will order them:

    foo.bar
    foo/bar

While (2) will order them:

    foo/bar
    foo.bar

The primary use case I know of for operator<() is default ordering for 
paths used as keys in associative containers. I can't see that either 
approach is superior for this use, so unless someone comes up with a 
compelling argument, (1) will be used.

Equivalence
-----------

Two paths will be considered equivalent if they resolve to the same 
physical directory or file.

Question 1: What is a use case that requires this function? Verifying that 
source and target files are different before some modifying operation is 
the only one I've come up with. I guess that is sufficient to justify 
adding the function.

Question 2: What if neither exist? Only one exists? My initial thought is 
that these are likely to be errors, so treat them as such. It could be 
argued that if either or both don't exist, they can't be equivalent, so 
return false.

Question 3: The implementation on Windows (see below) leaves a small hole 
in that duplicated media (such as two CD's) mounted on devices with the 
same device id on two different networked machines would be reported as 
equivalent. POSIX requires that such networked devices have different 
device id's, avoiding the problem. Is the fact that Windows and POSIX 
implementations would perform slightly differently on this corner case a 
showstopper? I think not.

Windows logic for path equivalent: same device id AND same media volume 
serial number AND same physical location on disk AND same creation time. 
This works even in degenerate cases like camera formatted FAT flash memory 
cards or floppy disks with volume serial numbers incorrectly initialized to 
0.

POSIX logic: same device id AND same physical location on disk AND same 
modification time. The modification time is in theory redundant, but is an 
added protection in case the device id on networked devices failed to meet 
the POSIX specs.

--Beman